Active magnetic regeneration combines a regenerator with a device which operates on the magnetocaloric effect. The operation of active magnetic regenerators is described in U.S. Pat. No. 4,332,135 to Barclay, et al. An experimental model of an active magnetic regenerator has been built and tested and is described in an article by A. J. DeGregoria, et al., "Test Results of An Active Magnetic Regenerative Refrigerator," Advances in Cryogenic Engineering, Vol. 37B, 1991. A detailed model of the active magnetic regenerator is given in an article by A. J. DeGregoria, Advances in Cryogenic Engineering, Vol. 37B, 1991.
An active magnetic regenerator is a type of cooler or heat pump which utilizes the magnetocaloric effect. Materials which exhibit the magnetocaloric effect warm upon magnetization and cool upon demagnetization, or vice versa. In a basic active magnetic regenerator (AMR) device, a bed of magnetic material which is porous to a heat transfer fluid is located between two heat exchangers, with a mechanism provided for effecting reciprocating fluid flow through the bed of magnetic material from one heat exchanger to the other. A mechanism is also provided for magnetizing and demagnetizing the bed. There are four parts to an AMR cycle: bed magnetization, which warms the magnetic material and the fluid in the bed by the magnetocaloric effect; cold side to hot side fluid flow through the bed with release of heat through a hot side heat exchanger; bed demagnetization, cooling the magnetic material and the fluid in the bed; and hot side to cold side fluid flow through the bed, with the cooled fluid absorbing heat at the cold side heat exchanger.
An AMR device is an extension of the regenerator concept. A regenerator is used to recover heat when fluid is exchanged in a reciprocating fashion between two reservoirs of different temperatures. The regeneration cycle has two parts: flow from the cold reservoir to the hot reservoir through the bed, followed by a flow from the hot reservoir to the cold reservoir through the bed.
In a regenerator device, the shuttle fluid is the total fluid mass which flows in one direction prior to reversal. After many reciprocating flows of the shuttle fluid through the bed, the bed material establishes a temperature profile which increases from the side at which the cold fluid enters (the cold side) to the side at which the hot fluid enters (the hot side). During the flow from the cold side to the hot side, the fluid enters at a temperature T.sub.c, the temperature of the fluid in the cold side reservoir. The shuttle fluid is warmed by the bed as it passes through the bed and leaves the bed at a temperature below T.sub.h, the temperature of the hot side reservoir. During the flow from the hot side to the cold side, the fluid enters the bed at the temperature T.sub.h, and is cooled by the bed as it passes through, leaving the bed at a temperature above T.sub.c. Over the entire cycle, the bed theoretically receives no net heat. It acts as an intermediate heat reservoir, absorbing heat from the warm fluid and rejecting it to the cool fluid. The difference in temperature between the temperature at which the shuttle fluid enters the cold reservoir and the temperature T.sub.c of the cold reservoir fluid, .DELTA.t, represents heat flow from the hot reservoir to the cold reservoir. At worst, this difference would be T.sub.h -T.sub.c, which is the case if there is no regenerator present. The ratio of .DELTA.t to (T.sub.h -T.sub.c) is referred to as the regenerator ineffectiveness.
An AMR device magnetizes and warms the bed prior to fluid flow from cold to hot, and then demagnetizes and cools the bed prior to flow from the hot side to the cold side. The application of the magnetic field to the magnetized bed creates a pair of profiles of temperature and relative position in the bed, one when the bed is magnetized and the other when the bed is unmagnetized. The difference between the two bed profiles at any location is the adiabatic temperature change of the magnetic material in going through the change in magnetic field. If the adiabatic temperature change is large enough, the fluid emerging from the cold side of the bed can have a temperature which is lower than the temperature of the cold reservoir, resulting in net cooling of the cold reservoir, rather than a heat leak from the hot reservoir to the cold reservoir which would be the case with an ordinary regenerator. Of course, in accordance with the laws of thermodynamics, work must be done in such a process since heat is flowing from a cold to a hot reservoir. In the case of an AMR, the work is performed by the drive mechanism which moves the magnet and/or the bed relative to one another, or by the use of a rotating permanent magnet or an electrically switched magnet. By utilizing the heat exchangers at both the hot side and the cold side, heat can be removed from the cold side heat exchanger through the AMR and released through the hot side heat exchanger. A structure for accomplishing this transfer is disclosed in the aforesaid U.S. Pat. No. 4,332,135.
The AMR cycle described above is similar to the Brayton cycle for a gas refrigerator in that the magnetization and demagnetization (work input) parts of the cycle are done at constant entropy, i.e., without fluid flow and heat transfer. In a magnetic analogue of the Ericsson cycle, the change in field is done at constant bed material temperature, which requires heat transfer during the magnetization and demagnetization processes. Cycles intermediate between the Brayton and Ericsson can also be performed, which are characterized by an amount of heat transfer during magnetization intermediate between the amounts required by Brayton and Ericsson cycles. AMR cycles which are analogous to Ericsson and intermediate cycles may be performed by allowing some fluid flow in the bed material during the magnetization and demagnetization process.
A further extension of active magnetic regenerators is shown in U.S. Pat. No. 5,249,424 to DeGregoria, et al., in which the flow of heat transfer fluid through the bed is unbalanced so that more fluid flows through the bed from the hot side to the cold side of the bed than from the cold side to the hot side. The excess heat transfer fluid is diverted back to the hot side of the bed, and multiple stages of active magnetic regenerators may be used. As described in this patent, the regenerator beds may be moved in and out of the magnetic field either in a reciprocating fashion or the beds may be mounted in a rotating wheel.
One of the disadvantages of active magnetic regenerators is the inefficiency encountered because the heat transfer fluid in reciprocating active magnetic regenerators is shuttled back and forth between the regenerator bed(s) and the respective hot and cold heat exchangers. Because the flow of fluid is not in a single direction between the beds and the heat exchangers, some amount of the heat transfer fluid is always in the connecting lines between the beds and the heat exchangers and never cycles both through the beds and the heat exchangers. This trapped heat transfer fluid, commonly referred to as the "dead volume," is a significant source of inefficiency in previous active magnetic regenerators.
In conventional gas cycle refrigerators, many of the most common and economically suitable refrigerants, such as chlorofluorocarbons, are environmentally hazardous. In magnetic refrigerators, including active magnetic regenerators, the working material is a solid, and a separate fluid is used to convey heat to and from the heat exchangers. Because the heat transfer fluid does not need to undergo compression and expansion, any fluid having acceptable heat capacity and flow characteristics over the temperature range of the refrigerator can be used.